1. Technical Field
The present invention relates to a wireless transponder and a wireless response method for transmitting/receiving a signal to/from an interrogator, and particularly to a wireless transponder capable of communication in a high-noise environment without a specific energy source, and an image forming device using the wireless transponder as a path sensor for a recording material.
2. Related Art
Various sensors have been developed with a view to measuring physical and chemical characteristics, and the like. In most such sensors, output signals vary depending on physical and chemical characteristics of the targets to be measured, and also the environment. Information such as physical and chemical characteristics concerning targets to be measured and the like is obtained by processing signals that are output as a result of measurement. The sensors are provided separately from a device (hereinafter an “interrogator”) for processing output signals from the sensors. Usually, the sensors and the interrogator are used at different respective locations. In such a case, it is necessary to provide signal cables from the sensors to communicate with the interrogator.
However, the structure of signal cables that are used for such a purpose gives rise to a problem, in that it becomes difficult to provide a signal cable to an object in motion such as a moving or rotating object, inside a very small space such as a blood vessel, or in a chemically or biologically harsh environment.
To address the problem stated above, signals may be wirelessly transmitted/received to/from an interrogator. A sensor used for this purpose is a wireless transponder called a surface acoustic wave sensor (SAW sensor) which uses a surface acoustic wave element (see Patent Documents 1-3). The surface acoustic wave sensor is a device which requires no specific energy source. The surface acoustic wave sensor may sense information concerning a target to be measured and an environment, and may send as a response an identification signal of a device.
Patent Document 1:U.S. Pat. No. 3273146Patent Document 2:JP-A-55-46159Patent Document 3:JP-B-3315984
However, wireless transponders described in Patent Documents 1-3 have a structure which includes a surface acoustic wave element which requires use of a high-frequency wave of several MHz or higher to transmit/receive a signal. Consequently, in a case that the wireless transponders used are surrounded by metal, the electromagnetic waves are not able to satisfactorily travel within a range of several cm (centimeters) or so due to the short length of the electromagnetic waves. Examples of such objects are electronic devices having a component in which a large number of conductive wiring patterns are formed, various manufacturing devices constituted of metal carriers, or vehicles and airplanes having metal frames. That is, there exists a problem that the wireless transponders do not function satisfactorily.
In another case of using an electromagnetic wave having a low frequency of several 100 kHz, a wavelength of the electromagnetic wave is as long as several km (kilometers) or more; but in this case, a problem arises in that their use makes it difficult to form a small antenna of a radiative emission type. An inductive emission antenna of a wound coil type may be employed with a view to downsizing an antenna to be used. However, when used in an environment that is surrounded by metal, smooth propagation of signals is hindered by absorption of magnetic fields, as mentioned above.